U.S. Pat. No. 4,816,784 to Rabjohn discloses a conventional cross over layout. Such a layout generally comprises coupling between adjacent windings (as shown in FIG. 3a and discussed on col. 4, line 55 of the issued patent). In addition, the crossovers by themselves constitute a parasitic which may degrade coupling performance.
Col. 7, line 22 of Rabjohn describes an asymmetry problem. A solution discussed by Rabjohn (starting on col. 7, line 31) involves floating or opening the conventionally grounded terminal of the primary winding. Rabjohn grounds the center tap of the primary winding to induce a symmetrically identical signal to the parasitic capacitor 40 as was originally applied to the parasitic capacitor 38 in FIGS. 6/7 of Rabjohn. By grounding the center tap of the primary winding, broadband Marchand operation is fundamentally non-existent. Furthermore, Rabjohn intentionally grounds the center tap of the secondary winding, eliminating the provision of an intermediate frequency (IF) center tap. In the ideal Marchand balun, the center tap port would physically be grounded disallowing the use of an IF center tap.
Multi-decade microwave mixers are needed for broadband communication systems. Such mixers range from a few MHz up to microwave frequencies at 20 Ghz or more. Inexpensive passive monolithically integrated microwave circuit (MMIC) Schottky mixers are attractive for their low parasitics and resulting wide frequency bandwidth, good intermodulation performance, and low cost. Some of the best conventional radio frequency (RF) MMIC mixer performance has been obtained from planar Marchand balanced mixers. However, such mixers are prohibitively large, requiring lambda/4 passive geometries.
Conventional active MMIC mixers can obtain multi-decade bandwidths, low power and high gain. However, such mixers consume significant DC power and cannot match the RF linearity of passive MMIC mixers. Wide band MMIC RF mixers have been implemented, but lack bandwidth, do not provide a small size/low cost, and/or have poor RF linearity.
MMIC Planar transformer Schottky mixers achieve excellent linearity using a reasonably size, but typically have sub-octave bandwidths. Active MMIC mixers, such as the Gilbert multiplier, achieve wide multi-decade bandwidth in a compact area. However, such implementations cannot match the RF linearity of passive mixer approaches.
MMIC lumped element passive Schottky mixers can achieve good RF linearity but are usually not as compact as the planar transformer approach. However, such mixers are also limited to about an octave in bandwidth. Marchand MMIC balanced Schottky mixers achieve some of the widest bandwidths and good Schottky linearity performance. However, they typically are large and do not offer a straightforward way of providing an IF center tap due to their inherent physical structure.
Referring to FIG. 1, a circuit 10 illustrating conventional transformer balun representation is shown. The transformer balun 10 is typically implemented as interwound planar microstrip transmission lines on a substrate. 1:1 transformer ratios are used to provide an even power split to the complementary outputs. Typical use of the transformer in mixers uses the primary coil as the single-ended input with one terminal receiving the signal and the other terminal being grounded. A secondary coil provides complementary (i.e., 0 and 180 degree) outputs. A center tap can be taken from the middle of the secondary coil. The center tap combines signals introduced from the complementary ports in a destructive manner to provide inherent isolation from the input and complementary ports.
Referring to FIG. 2, a circuit 20 illustrating a conventional MMIC implementation of a planar transformer balun on a GaAs Substrate is shown. The planar center tapped balun consists of a pair of interwound planar microstrip transmission lines. Suggested dimensions of the coil is given for a 100 um thick Gallium Arsenide (GaAs) implementation for microwave operation of 3–9 GHz for a grounded balun, and 4–20 GHz for an open balun. The primary coil has a single-ended input where the other side is either grounded (conventional case) or open (Marchand applications). The secondary coil provides the complementary output and center tap. In conventional use, one end of the primary coil is grounded.
Referring to FIG. 3, a conventional center tap transformer 30 is shown. With 50 ohms at the input and output ports, the balun transformer response for the practical planar transformer 30 will be dependent on the impedance at the center tap port. One characteristic found in the planar transformer balun is that transmission line characteristics are dependent on port impedances. More particularly, the planar transformer balun has a tendency to have better complementary matched outputs when the center tap impedance is zero. For the local oscillator (LO) port of a mixer, the center tap is normally grounded, providing excellent balanced performance and LO isolation at the center-tap. For the RF port where the center tap is used as the IF port, the center tap is not typically grounded. Such an RF port is typically matched to a 50 ohm IF amplifier load, making the RF transformer operation less ideal over a broad frequency range.
Referring to FIG. 4, electromagnetic (EM) simulations of “Short” planar transformer amplitude and phase balance is shown. Amplitude and phase balance become worse as Ztap becomes larger. FIG. 4 shows the EM simulation of a planar balun transformer (similar to the construction of FIG. 2) for two different IF center tap impedances (i.e., Ztap=10 ohms and Ztap=50 ohms). Comparing the amplitude and complementary phase balance for the two cases, the amplitude and phase balance (180 degrees) between the complementary outputs of the balun are better matched over the 2–8 GHz frequency range for the 10 ohm Ztap impedance case. The amplitude balance of FIG. 4a rolls off slower than that of FIG. 4c of the 50 ohm case. Also the complementary phase balance of FIG. 4b is flatter across the frequency band than that of FIG. 4d of the 50 ohm case. The need for lower impedance becomes even more dramatic for open balun transformer implementations.
Referring to FIG. 5, a circuit 50 illustrating a conventional planar transformer open configuration is shown. By opening the grounded port of the planar transformer, a wider band high pass Marchand balun can be created due to the transmission-line properties of the traces making up the coils. In essence, this configuration resembles a Marchand balun. Modeled as an ideal transformer in a microwave simulator (ADS), this wide band operation is not observed due to the absence of the transmission-line construction.
The circuit 50 illustrates a Marchand balun application of the planar transformer for achieving a broader and high pass bandwidth using the same construction as the circuit 20 of FIG. 2. With the circuit 50, the terminal which is conventionally grounded is now opened. Because of the transmission line properties of the microstrip coils, the circuit 50 produces a multi-octave high-pass frequency response and operates as a Marchand balun. The circuit 50 can be applied to a monolithically integrated microwave circuit (MMIC) for implementing an inexpensive microwave mixer. The circuit 50 provides bandwidth comparable to conventional coupled line Marchand MMIC mixers, while being implemented smaller and more affordably due to the compact nature of the planar balun structure. Such an implementation is discussed in the articles (i) Steve Maas, et al., 1996 MMMCS, pp. 51, (ii) S. A. Maas, et al., 1993 MMMCS, pp. 53, 2.4×2.4 mm2, and (iii) Y. Ryu, et al. 1995 MMWMCS, which are each incorporated by reference in their entirety. As in the conventional grounded balun configuration, lowering the impedance of the center tap improves the balanced performance over a wider frequency of operation. The improvement in balanced operation in an open configuration is more dramatic than in the conventionally grounded transformer configuration 30.
Referring to FIG. 6, an EM simulations of an “Open” Planar Transformer Amplitude and Phase balance is shown. The open planar balun obtains multi-octave bandwidth. Furthermore, the impedance of the center tap has a similar effect as a conventional grounded transformer. In particular, the transformer becomes more unbalanced as Ztap increases.
FIG. 6 illustrates the amplitude and complementary phase balance of the open planar transformer for a center tap impedance of 10 ohms and 50 ohms. For the 10 ohm Ztap case, amplitude and complementary phase balance illustrated in FIGS. 6a and 6b are maintained over a 6–20 GHz multi-octave frequency band. FIGS. 6c and 6d illustrate the 50 ohm Ztap case. The improvement in balun performance is more dramatic for this planar spiral Marchand (open) configuration than for the conventional grounded transformer case of FIG. 4.
Referring to FIG. 7, a circuit 70 illustrating a conventional transformer balanced Schottky mixer is shown. Optimal wideband planar Marchand transformer and consequently overall mixer performance can be obtained by integrating a low impedance actively matched IF buffer amplifier at the IF center tap of the RF transformer balun. The circuit 70 illustrates a conceptual schematic of a typical transformer balanced Schottky mixer where the RF transformer has an IF center tap. In order to optimize the RF planar transformer balun and overall mixer performance, a low impedance actively matched IF buffer amplifier can be applied to the IF center tap of the RF transformer balun.
It would be desirable to implement loading of a center tap with an optimal finite impedance, such as an active impedance that is electronically tunable. It would also be desirable to implement a MMIC mixer with the RF performance of a Marchand balanced approach, but with the wide bandwidth, gain, and compact size of an active approach. It would further be desirable to provide a broadband low impedance to the center-tap of the transformer balun for optimal broadband balun performance.